The invention relates to an optical element comprising a transparent substrate provided with an antireflection coating for the range of wavelengths from approximately 700 nm to approximately 1700 nm. The coating comprises a top coating and a base coating. The top coating has a plurality of uniform layers whose refractive indices decrease toward the top of the coating.
The optical element may be an element of an optical fiber telecommunication system. It may be, for example, a lens or the end face of a fiber. In this respect, fiber is to be understood to mean a light-conducting fiber with a cladding. The optical element may also be an element of an infrared scanning system for medical or military use.
British Pat. No. 1,380,793 describes an antireflection coating which is suitable for visible wavelengths, i.e. for the range from approximately 400 nm to approximately 800 nm. This coating comprises five superimposed homogeneous layers having different refractive indices. Each layer has an optical thickness equal to .lambda..sub.o /4, .lambda..sub.o being a reference wavelength. The first layer on the substrate has a refractive index which is greater than that of the substrate. The second layer deposited on this first layer has a refractive index which is greater than that of the first layer. These two layers form the base coating. The next three layers form the top coating and have refractive indices which decrease toward the top of the coating.
In optical fiber-based telecommunication systems, it is necessary to eliminate reflections over a wide range of wavelengths from approximately 800 nm to approximately 1650 nm. This is because in optical telecommunication systems two basic types of light sources are used: diode lasers of the GaAs-type, which emit wavelengths in the range from approximately 800 nm to approximately 900 nm, and diode lasers of the InP-type, which emit wavelengths in the range between approximately 1100 nm and approximately 1650 nm. The reflection coefficients of the elements in optical telecommunication system should preferably be 0.5% or less.